US3616308A - Method of producing colored coatings on aluminum - Google Patents

Abstract

In procedure for producing colored coatings on aluminum by first anodizing the aluminum surface and then treating such anodically coated surface with alternating current in an acidic bath containing metal ions selected from the group consisting of the following cations and anions: Cu , Ag , Pb , and anions consisting of oxygen combined with one of the metals Se, Te and Mn, to produce a colored deposit in the coating, improved coloring results notably in avoiding or inhibiting occurrence of localized nonuniformity, especially with darker or more intense tones, are achieved by maintaining a content of aluminum ions in the bath.

Description

United States Patent [72] Inventors William Ernest Cooke;

Paul John Sajben; Roy Cowieson Spooner, all of Kingston, Ontario, Canada 21 Appl. No. 773,936

[22] Filed Nov. 6, 1968 [45] Patented Oct. 26, 1971 [73] Assignee Alcan Research and Development Limited Montreal, Quebec, Canada [32] Priority Nov. 24, 1967 [33] Great Britain [54] METHOD OF PRODUCING COLORED COATINGS 0N ALUMINUM 13 Claims, No Drawings [52] U.S. Cl 204/58 [51] lnt.Cl C23b 9/02 [50] Field of Search 204/58, 35

[56] References Cited UNITED STATES PATENTS 3,382,160 5/1968 Asada 204/58 2,930,951 3/1960 Burger et al. 204/58 FOREIGN PATENTS 4/1963 Canada 11/1943 Germany OTHER REFERENCES The Surface Treatment of Aluminum, by Wernick & Pinner, 3rd ed., 1964, p. 348

ABSTRACT: In procedure for producing colored coatings on aluminum by first anodizing the aluminum surface and then treating such anodically coated surface with alternating current in an acidic bath containing metal ions selected from the group consisting of the following cations and anions: Cu, Ag", Pb, and anions consisting of oxygen combined with one of the metals Se, Te and Mn, to produce a colored deposit in the coating, improved coloring results notably in avoiding or inhibiting occurrence oflocalized nonuniformity, especially with darker or more intense tones, are achieved by maintaining a content of aluminum ions in the bath.

METHOD OF PRODUCING COLORED COATINGS ON ALUMINUM BACKGROUND OF THE INVENTION The present invention relates to a process for producing colored coatings on aluminum, and more particularly relates to the production of inorganic-colored coatings on aluminum articles. References to aluminum herein will be understood to include aluminum of ordinary commercial purity and aluminum-base alloys that are suitable for conventional anodizing treatments. The term aluminum article as used herein is intended to include semifabricated products, such as rolled aluminum sheet and aluminum extruded sections.

The present invention relates to improvements in a previously described two-stage process for producing a colored coating which comprises anodically oxidizing an aluminum article and thereafter treating the anodized article by passing an alternating current between such aluminum article and a counterelectrode, while such aluminum article and electrode are immersed in a bath that contains a dissolved compound of a selected metal in an aqueous acidic medium. Under these circumstances the anodic oxide coating on the aluminum article becomes colored. More particularly, the invention is con cerned with such process wherein the acidic bath of the alternating current treatment contains metal ions selected from the group consisting of the following cations and anions: Cu, Agfl Pb. and anions consisting of oxygen combined with one of the metals Se, Te and Mn. Procedures of this sort are included in the description of U.S. Pat. No. 3,382,160, granted May 7, 1968 to Tahei Asada.

in the above, known process in which the aluminum article is in the first stage subjected to anodizing, preferably with direct current, for example in a sulfuric acid electrolyte, i.e. under conditions known for the production of an anodic oxide film of the so-called porous type, the counterelectrode in the alternating current step may be a material, such as graphite (i.e. carbon), which is inert to the electrolyte, or may be formed of other conductive composition, e.g. metal, which may advantageously be selected among metals having a solution potential more noble than aluminum in the conventional series of electrode potentials, and which can preferably be the same metal, if available, as constituted by the selected ions in solution in the electrolyte, all as explained in the previous disclosure. For instance, where the bath electrolyte contains copper dissolved therein, a graphite or copper counterelectrode may be effectively employed.

As stated, it is known by the foregoing procedure to produce colored deposits in the anodic coating, being understood to be a deposit of the selected metal in chemical combination with oxygen, i.e. a compound of the class of oxide or hydroxide, it having been further indicated that depending on the selected metal and intensity or duration of r'eatment, a variety of colors are obtainable, including gold or other yellow tones, various shades of red, maroon, brown, dark brown and like deep colors, including black.

in the known process certain difficulties have been experienced especially when the treatment time or conditions are adjusted to give darker shades, associated with a relatively large or intense deposition of the metallic material, presently understood to be metallic oxide, in the anodic oxide film on the aluminum. in particular there is an increased tendency to so-called spalling, i.e. local flaking or like modification of the anodic oxide film, this effect being to produce colorless or pale local spots in the coating, which may be of minute size but are nevertheless extremely objectionable.

A chief object of the present invention is to reduce the incidence of spalling. A further and more specific object is to improve the operation of the above process utilizing metal ions selected from the stated group, notably the second or alternating current treatment stage of such process, so that a full range of tones or shades may be obtained by employing a given bath, including the darker tones that are achieved with higher voltages or longer times or both, while in all cases there is desirable avoidance of deterioration of the colored coating as by spalling or the like.

SUMMARY OF THE INVENTION With one or more of the foregoing aims in view, it is found that more uniform results, and specifically an inhibition or avoidance of spalling and like deterioration, can be obtained by the introduction of aluminum as in the form of aluminum compounds, into solution in the electrolyte, i.e. the acidic bath employed in the alternating current step. More particularly a significant content of aluminum ions (AI Z herein designated Al) is achieved and maintained in the bath, as by incorporating an aluminum salt such as aluminum sulfate or other appropriate compound, the process being otherwise generally performed as heretofore disclosed. Under these circumstances relatively very dark or intense shades or tones can generally be achieved without spalling or like deterioration; or at least, it is possible to obtain darker shades, without the risk of spalling, than has heretofore been uniformly or reliably attainable.

The complete procedure includes first anodizing the aluminum surface to produce the anodic coating thereon, and then subjecting the coated surface to alternating current in an aqueous bath containing the selected metal ions, such bath also containing aluminum ions, as for example in amount represented by at least about I gram per liter of aluminum sulfate (calculated as Al,[SO,],.l8l-l,0), or more generally (and approximately) stated at least about ppm. Al ions. Thus a typical bath may constitute the salt of the metal selected for coloring function, appropriate acidic and related constituents, and an aluminum compound which in dissolved form is in effect an aluminum salt, advantageously containing an anion otherwise present in the electrolyte.

The underlying, known method of producing colored coatings has been found of special practical value as utilizing copper-containing baths, and it has correspondingly been noted that such operations are markedly improved by the maintenance of combined aluminum in solution. A special advantage of the present invention is that a given electrolyte can be employed to provide a wide range of shades or tones, notably up to the darkest values, without difficulty, so that the process has improved flexibility in practice, to accommodate a wide variety of color requirements. Moreover, it has been noted, for example in the copper'containing, lead-containing and selenium-containing solutions, and others, that more intensely colored and spall-free surfaces are obtainable from the aluminum-bearing electrolyte under otherwise identical conditions.

DETAILED DESCRlPTlON As stated, the complete process of producing a colored coating on an aluminum surface of an article involves first anodizing the surface in conventional manner to produce an anodic oxide coating, e.g. of a type customarily applied for protective or like purposes. While any of a number of known operations may be employed, notably with electrolytes of a group which may be defined as consisting of aqueous solutions of sulfuric acid, chromic acid, or a sulfonic acid such as sulfosalicylic acid, and suitable mixtures of these with other acids or compounds, and while in some cases alternating current anodizing treatment may be feasible, effective results are obtained by anodizing the work with direct current, as for periods of 20 minutes to 60 minutes, in an aqueous solution of sulfuric acid, e.g. 15 percent acid by weight. The operating conditions of the anodizing step do not appear to be very critical, being selected largely to suit the thickness and other characteristics of anodic coating desired; the requirements of the subsequent coloring step are satisfied over a considerable range of thicknesses of porous oxide coating on aluminum,

The second step of the process, to which the present im provements are specifically directed, then involves submerging the anodized aluminum article, if desired afier suitable rinsing, in an acidic bath in which a counterelectrode is likewise submerged, such bath comprising an aqueous acidic solution containing the metal selected for coloring function, specifically metal ions selected from the group consisting of the following cations and anions: Cu, Ag Pb, and anions consisting of oxygen combined with one of the metals Se, Te and Mn. Thereupon alternating current is passed through the solution between the article and the counterelectrode, for sufficient time to effect the desired colored deposit in the oxide coating, e.g. a deposit understood to consist of metal, of the selected ions, which is in chemical combination with oxygen, such as oxide or hydroxide, conveniently herein considered to be oxide.

It is in this acidic bath that a content of aluminum ions is maintained in accordance with the present invention, preferably by adding a quantity of suitable aluminum salt (or other compound which becomes dissolved) in substantial excess so as to assure suitable presence of such ions from the very outset and as the bath is thereafter repeatedly used.

As will be apparent from the prior disclosure mentioned above, the basic requirement of the process is that the bath should be a water solution containing the metal ions selected for coloring function, and an acidic constituent, the dissolved compounds being all in relatively low concentration. Thus the bath constituents, including nonmetallic anionic constituents, may be variously selected to provide the desired solubility of metal ions and suitable acidity of the electrolyte, and supplemental or further salts and the like may be incorporated, as desired for supplemental purposes that do not interfere with the formation of the colored deposit, or as may result from initial or subsequent pH adjustment. Thus sulfate, borate, acetate, tartrate, phosphate, sulfamate and other common ions have been found appropriate for solutions of one metal or another, e.g. with respect to salts of the selected metal or as forming the chosen acidic component. It will be readily apparent that the constituents will depend on solubility and like requirements of the metal ions, as for example in the case of lead ions, substances such as sulfamic acid and acetic acid and their salts should be utilized instead of sulfate. Where the metal ions are of anionic nature, compounds appropriately soluble in the acidic bath should be employed, such as alkali metal selenites, selenium oxide, selenic acid, alkali metal tellurites, permanganates, and the like.

Examples of suitable metal-containing baths and types of compounds therein are set forth in the above-cited US. Pat. No. 3,382,160, to which reference may be had for such information, as indeed likewise for inherent or other indication of generally preferable conditions of acidity. For instance, notably good results have been attained in baths containing metal ions (whether cations or anions) of the group to which the present invention is applicable, with the pH adjusted to the range of about 0.5 to about 2, it being noted that if the pH rses significantly above 4.5, combined aluminum (added in accordance with the present invention) begins to precipitate out.

For the purposes of this invention, aluminum is preferably added to the electrolyte in the form of a salt, the anion of which (e.g. sulfate, acetate) is already present in the bath or is otherwise compatible as will be understood or may be readily determined. Thus for example a presently preferred copper electrolyte contains copper sulfate and sulfuric acid, to which the aluminum may be added as aluminum sulfate. Alternatively, the aluminum can actually be supplied as a soluble form of aluminum .oxide or aluminum hydroxide, then becoming a corresponding dissolved salt in the presence of acidic constituents. Necessary adjustments of acidity for this and other purposes are made by suitable additions such as sulfuric acid, acetic acid, ammonium hydroxide and the like. While some anions, such as halides (except perhaps chlorides in small concentration) are not presently recommended, the essential requirement is to have dissolved aluminum ions present and selection of a suitable and commonly available salt presents no problem in the light of the foregoing.

In general the amount of aluminum present should be sufficient to provide significant inhibition of spalling, i.e. to avoid or materially reduce the occurrence or tendency toward occurrence of minute spots or flaked-off areas, or like spots or defects in the colored product, e.g. in the darker or very dark tones obtainable from the selected metal. Although such spalling is often represented by colorless or pale spots, it may sometimes occur otherwise, as for instance where flaking near the outset of the alternating current treatment leads to a more intense tone due to preferential flow of current to the spalled area. Although some utility has been noted for lesser amounts, in general, at least about p.p.m. of aluminum (measured as such, by weight) should be present for good results, and indeed preferably at least somewhat more, such as represented by about 2 grams per liter of conventional aluminum sulfate (approximately 200 ppm. al.). Preferably the aluminum compound is added in amount to provide from approximately 1000 to 2500 p.p.m. of al., as for example about 10 to 25 grams per liter of AL ,(SO 1 811,0. Even considerably greater amounts, as up to 40 grams per liter of the aluminum salt (or 4000 ppm. Al), are not believed to be detrimental, so that the upper limit appears primarily economic.

The exact effect of adding aluminum sulfate or other aluminum compound to the electrolyte is not known. However, a possible belief is that spalling is due to local differences in the electrical resistance of the anodic oxide film, or more particularly that spalling may be related to the intense evolution of hydrogen where the oxide layer at the bottom of one or more pores of the coating is thin in a local area. It is believed that the incorporation of aluminum sulfate, for example, in the electrolyte leads to some codeposition of aluminum oxide with the colored oxide or the like in the pores of the anodic oxide film and that such deposition increases the thickness of electrically resistant oxide at the bottom of the pores, thus removing the postulated cause of spalling and permitting the development of darker colors therein. Stated in another way, this hypothesis is that aluminum oxide or hydroxide is coprecipitated with the colored oxide or hydroxide of the metal of which ions are present from the above-defined group, to provide a more electrically resistant deposit, a present theory being that the relatively larger flow of current during that part of the alternating current cycle when the anodized article is the cathode results in depletion of hydrogen ions at the base of the pores, causing localized rise in pH (greater alkalinity) and consequently the desired precipitation of the metal oxide or hydroxide, and in the present process, of aluminum oxide or hydroxide as well. However, the beneficial effects of this invention have been abundantly demonstrated and it is not to be understood as dependent on the above or any other theories.

Referring to the following specific examples of the process, aluminum articles were first anodized by conventional anodic treatment with direct current, e.g. for periods upwards of 20 minutes in a 15 percent aqueous sulfuric acid solution at an ordinary temperature, for example selected in the range of 20 C. to 25 C., conveniently 2l C. It will be understood that other strengths of acid solution, or indeed solutions of other acids as mentioned above, and likewise other modifications of condition as to suit the protective or like requirements of the coating may be employed. Conveniently operation under the first named conditions, with a current density of about 15 amperes per square foot (1.3 amperes per square decimeter) of work surface is very satisfactory, for a time selected to yield a desired film thickness, normally from 0.4 to L0 mil or more. As will be understood, the anodized article is then suitably rinsed and submerged in the selected acidic bath which contains the appropriate counterelectrode. In general, the alter nating current is passed, between the article and the electrode, at a conveniently moderate voltage, say from 5 to 20 volts, for a correspondingly appropriate time, e.g. from a few minutes up to 10 or i5 minutes, sometimes with increasing voltage or other suitable electrical control. Generally, the shade of color achieved depends on the time of treatment and on electrical conditions; for instance, lighter shades are produced by relatively brief treatment with lower voltages, whereas darker or stronger or very dark colors require a longer time or greater electrical values.

EXAMPLE 1 Aluminum articles, e.g. aluminum sheets were first anodized as above in the conventional 15 percent sulfuric acid solution for 30 minutes to a film thickness of 0.6 mil, and after rinsing were then subjected to the alternating current treatment, with a counterelectrode of graphite in a copper-containing bath (adjusted to pH about 1.3) having the following composition:

Aluminum articles, as anodized, were treated in this bath for a total time consisting of 3 minutes at volts followed by 9 minutes at volts, yielding a practically black color of good uniformity, i.e. free of spalling or other localized nonuniformity, In this and in other examples herein, the treated articles can preferably be subjected to conventional sealing treatment, as in very hot or boiling water, for usual purposes. For comparison, operation exactly as above but with the bath lacking the aluminum sulfate addition produced a coating which was considerably spalled.

It will be understood that at lower voltages or shorter times or both, lighter colors are obtainable, e.g. ranging through pink, light maroon and dark maroon, up to black, depending on the extent of electrical treatment. In practice, the content of copper salt can be selected over a considerable range, e.g. up to 50 g.p.l., the acid being as required to obtain a preferred pH and the aluminum sulfate being included in amounts, for example, from 2 to 30 g.p.l. or more.

EXAMPLE 2 Aluminum sheet was first anodized and then subjected to the alternating treatment as in example 1, except that the bath for the latter contained silver and had the following composition:

AgNO X L0 g.p.lr d ds lho g.p.l. H,SO, To provide pH 07 Passage of alternating current for 3 minutes at 10 volts yielded an article having a coating of uniform, golden brown color, free ofdefects.

EXAMPLE 3 Aluminum sheet was first anodized and then subjected to the alternating treatment as in example 1, except that the bath for the latter contained selenium and had the following composition:

Passage of alternating current for 10 minutes at l5 volts yielded an article having a coating of uniform gold color, free of defects. The color was significantly more intense than was obtainable by exactly the same treatment without the aluminum sulfate addition.

EXAMPLE 4 Aluminum sheet was first anodized and then subjected to the alternating treatment as in example l, except that the bath for the latter contained tellurium and had the following composition:

10 minutes at 15 volts yielded an article having a coating of uniform green-gold color, free of defects.

EXAMPLE 5 In this instance aluminum articles were first anodized by conventional treatment as indicated in other examples, and then subjected to the alternating current treatment, with a graphite counterelectrode, in a bath containing from 25 to 30 g.p.l. oflead acetate, 20 to 25 g.p.l. of acetic acid and 20 g.p.l. of aluminum acetate (Allc l-l o j Alternating current was passed at 13 volts for about 5 minutes. A uniform black coating was produced, whereas similar operation without the aluminum addition tended to exhibit some spalling or like difficulty.

Extended tests have also indicated that aluminum ion additions, for example by employment of appropriate aluminum salts in amounts of the order indicated in the above specific examples, are also advantageous for the purposes of the present invention, in the baths for alternating current treatment in each of those of the examples of the above-cited US. Pat. No. 3,382,160 which relate to the employment of metal ions (whether cations or anions) of the specific group hereinabove set forth.

It is to be understood that the invention is not limited to the particular compounds and operations herein set forth but may be carried out in other ways without departure from its spirit.

We claim:

1. In a method of producing an inorganically colored anodic coating on an aluminum surface of an article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting of the following cations Cu, Ag*, pb, and anions consisting of oxygen combined with one of the metals Se, Te and Mn, for producing a colored deposit in the coating, the procedure comprising adding to said bath an aluminum compound in an amount to provide in solution in said bath at least about ppm. of aluminum ions and effecting said alternating current treatment while maintaining said aluminum ion content of at least about 100 ppm. in said bath to inhibit occurrence of localized nonuniformity of the colored coating.

2. A method as defined in claim 1, in which the metal ions selected from said group consist essentially ofCu" ions.

3. A method as defined in claim 2, in which said step of adding the aluminum compound consists of adding aluminum sulfate to the bath in amount represented by at least about one gram of Al (SO -l8H O per liter.

4. A method as defined in claim 1, in which said step of adding the aluminum compound consists in adding an aluminum salt to the bath in amount of at least about 2 grams per liter.

5. A method as defined in claim 1, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating.

6. A method as defined in claim 5, in which in the bath through which alternating current is passed to said anodized surface the metal ions selected from said group consist essentially of Cu ions, and said aluminum compound is incorporated in the bath in amount to provide dissolved, combined aluminum equivalent to at least about 2 grams of Al (SO.) l8I-l O per liter.

In a method of producing an inorganically colored anodic coating on an aluminum surface ofan article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting anions consisting of oxygen combined with one of the metals Se, Te and Mn, for producing a colored deposit in the coating the procedure comprising adding to said bath an aluminum compound in an amount to provide in solution in said bath at least about 200 p.p.m. of aluminum ions and effecting said alternating current treatment while maintaining aluminum ions in said bath in amount of at least about 200 p.p.m.

8. A method as defined in claim 7, in which the metal ions selected from said group consist essentially of l 'b ions, said bath consisting essentially of lead acetate, acetic acid and aluminum acetate to provide said aluminum ions.

9. A method as defined in claim 7, in which the metal ions selected from said group consist essentially of Cu ions.

[0. A method as defined in claim 9, in which said bath consists essentially of copper sulfate, sulfuric acid and aluminum sulfate to provide said aluminum ions.

11. A method as defined in claim 10, which comprises adding Al,(SO 'l8I-i=O to the bath in amount of at least about l0 grams per liter to provide said aluminum ions.

12. A method as defined in claim 7, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating.

13. A method as defined in claim 12, in which said bath, through which alternating current is passed to said anodized surface, consists essentially of copper sulfate, sulfuric acid and aluminum sulfate, said bath being maintained at a pH between about 0.5 and about 2 and said aluminum sulfate providing at least about 200 p.p.m. of aluminum ions.

! I? i l II UNITED STATES PATENT OFFICE CERTIFICATE OF CORRE Patent No. 3,616 308 Dated October 26, 1971 Inventofls) WILLIAM ERNEST COOKE, PAUL JOHN SAJBEN and ROY COWIESON SPOONER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

C01 4 line 21 after "p.p.m "A1 should read --A1 Col. 5 line 45 (in table) in "X 1.0 g .p 1 delete "X" C01 6 line 4 insert Passage of alternating" current for-- at beginning of paragraph following table C01 6 line 70 before "In" insert 7.

C01 7, line 1 after "consisting" first occurrence) insert -of the following: cations Cu Ag Pb and-- Signed and sealed this 11 th day of April 1972.

(SEAL) Attest:

ILifgEJTCHER, JR. ROBERT GOTTSCHALK 5 mg ficer' Commissionerof Patents m po'wso USCOMM-DC 60375-P09 I u I nnvIInuI-nv o-nnmr. Arnen1. "-1"

Claims (12)

1. 2. A method as defined in claim 1, in which the metal ions selected from said group consist essentially of Cu ions.
2. 3. A method as defined in claim 2, in which said step of adding the aluminum compound consists of adding aluminum sulfate to the bath in amount represented by at least about one gram of Al2(SO4)3.18H2O per liter.
3. 4. A method as defined in claim 1, in which said step of adding the aluminum compound consists in adding an aluminum salt to the bath in amount of at least about 2 grams per liter.
4. 5. A method as defined in claim 1, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating.
5. 6. A method as defined in claim 5, in which in the bath through which alternating current is passed to said anodized surface the metal ions selected from said group consist essentially of Cu ions, and said aluminum compound is incorporated in the bath in amount to provide dissolved, combined aluminum equivalent to at least about 2 grams of Al2(SO4)3.18H2O per liter.
6. 7. In a method of producing an inorganically colored anodic coating on an aluminum surface of an article wherein said surface has first been anodized to produce an anodic coating thereon and wherein alternating current is passed between said anodized surface and an electrode while both are immersed in an aqueous acidic bath containing metal ions selected from the group consisting of the following cations cu , Ag , Pb and anions consisting of oxygen combined with one of the metals Se, Te and Mn, for producing a colored deposit in the coating the procedure comprising adding to said bath an aluminum compound in an amount to provide in solution in said bath at least about 200 p.p.m. of aluminum ions and effecting said alternating current treatment while maintaining aluminum ions in said bath in amount of at least about 200 p.p.m.
7. 8. A method as defined in claim 7, in which the metal ions selected from said group consist essentially of Pb ions, said bath consisting essentially of lead acetate, acetic acid and aluminum acetate to provide said aluminum ions.
8. 9. A method as defined in claim 7, in which the metal ions selected from said group consist essentially of Cu ions.
9. 10. A method as defined in claim 9, in which saiD bath consists essentially of copper sulfate, sulfuric acid and aluminum sulfate to provide said aluminum ions.
10. 11. A method as defined in claim 10, which comprises adding Al2(SO4)3.18H2O to the bath in amount of at least about 10 grams per liter to provide said aluminum ions.
11. 12. A method as defined in claim 7, which includes the step of first anodizing said aluminum surface of the article with direct current between said article as anode and a cathode in an aqueous solution of sulfuric acid to produce said anodic coating.
12. 13. A method as defined in claim 12, in which said bath, through which alternating current is passed to said anodized surface, consists essentially of copper sulfate, sulfuric acid and aluminum sulfate, said bath being maintained at a pH between about 0.5 and about 2 and said aluminum sulfate providing at least about 200 p.p.m. of aluminum ions.